In recent years atomic layer deposition (ALD) has been adopted as a manufacturing technique in several fields, including the semiconductor industry. ALD deposits films in a cyclical process, typically alternating exposure of the substrate to two or more reactants in phases separated in time and/or space, where each phase has a self-limiting effect. For example, one phase of the cycle can chemically adsorb, in a self-limiting fashion, a monolayer or less of a precursor or fragment thereof in each cycle. Often the adsorption is self-limited due to ligands of the precursor being inert relative to the adsorbed species, such that after the substrate surface is saturated the adsorption process stops. Reactants in a subsequent phase can react with the adsorbed species to remove the condition that limits the adsorption, such as stripping away ligands form the adsorbed species, for example by chemical reduction or replacement reactions, such that the precursor can again adsorb in a self-limiting fashion in a subsequent cycle. In one example, less than a monolayer of an organic silicon precursor can adsorb in a first phase, and an oxygen-containing reactant can strip the organic ligands from the adsorbed species and leave oxygen in a second phase.
More complicated ALD recipes may include three, four or more reactants, and relative frequencies of the phases may be adjusted to tune the composition of the layer being formed. Typically, each cycle leaves about a molecular monolayer or less per cycle. Many ALD processes average one monolayer every 3-10 cycles because of variety of reasons, such as steric hindrance from large precursor molecules prevents access to all reaction sites in a single cycle, lack or low number of reactive sites or other reasons. Mutually reactive reactants can be kept separated in time and/or space, e.g., by separating pulses by purging, or moving the substrate through different zones with separate reactants. However, variations on the process, such as schemes providing hybrid ALD and chemical vapor deposition (CVD) reactions, can obtain more than a monolayer per cycle.
Despite rather slow growth rates compared to traditional deposition techniques, such as sputter deposition and CVD, ALD has been growing in popularity for several reasons. For example, in the semiconductor industry, ALD can provide much greater step coverage, or conformal growth, or smoother or more uniform films over complex topography compared to other deposition techniques, particularly for very thin layers over structures with high aspect ratios. Need for such conformal layers tends to increase as circuits become more dense. Because the technique is self-limiting in each cycle, and because usually the growth rate tends to be independent of small temperature variations over a substrate, ALD offers almost perfect step coverage. Moreover, ALD tends to involve lower temperatures than other deposition techniques, which also becomes more important with successive generations of integrated circuits in order to conserve ever-stricter thermal budgets and preserve precise device junction depths. Similarly, ALD is increasingly attractive to other industries that could benefit from ultrathin, conformal and/or low temperature depositions.
To preserve self-limiting nature of ALD it can be important to prevent thermal breakdown of precursors that are meant to adsorb largely intact. Thermal breakdown can lead to time-dependent CVD growth mechanisms which can nullify the conformal deposition advantages of ALD. At the same time, some precursors demand significant energy to react with adsorbed species. Ensuring these competing conditions are satisfied can involve delicate trade-offs between substrate temperatures and prolonged exposures to ensure saturative surface reactions.
Plasma ALD processes, sometimes referred to as plasma assisted ALD or plasma enhanced ALD (PEALD), have been developed in order to improve the reaction energy of some phases without increasing the temperature of the substrate. For example, an organic or halide reactant can adsorb less than a monolayer in a first phase, and a second phase can expose the substrate to the products of a nitrogen-, hydrogen- or oxygen-containing plasma to strip ligands from the adsorbed species and/or leave nitrogen or oxygen in the film. However, in some cases it has been difficult to obtain high quality films using conventional plasma ALD techniques.